The present invention relates to a Fourier transformation infrared spectrophotometer.
In a Fourier transformation infrared spectrophotometer (which is hereinafter referred to as an FTIR), a fixed mirror and a movable mirror constitute a Michaelson Interferometer, which generates an interference wave whose amplitude changes with time. A sample is irradiated with the interference wave, and the transmitted light or the reflected light is detected as an interferogram. The detected interferrogram is Fourier transformed and an absorption spectrum is constructed with the wavenumber as an abscissa and the strength (the transmittance or the absorbance) as an ordinate.
In an FTIR, an absorption spectrum over an entire range of preset wavelengths can be obtained with a single reciprocal movement of the movable mirror. Since the S/N ratio is low with such a single movement, in general, the movable mirror is moved reciprocally several times and the data of the interferograms are accumulated. Then the accumulated data is Fourier transformed to generate an absorption spectrum with a high S/N ratio.
In the following cases, however, an error may occur in the interferogram of an FTIR.
(1) Just after the light source is turned on, the temperature of the light source is still changing and the strength of the light is not yet stable.
(2) Just after a sample is set in a sample chamber, the output of the detector (pyroelectric detector, for example) is not yet stable.
(3) When a strong shock is given to the apparatus, the movement of the interferometer is influenced by the shock.
(4) When an electrical noise intrudes the electrical circuit, a corresponding noise appears in the interferogram.
Since, in conventional FTIRs, data of the interferograms are accumulated irrespective of the quality of the interferograms, so that the final accumulated wave data is highly probably tainted. This causes (a) noises are superimposed on the absorption spectrum, (b) the baseline shifts, or in some cases (c) a peak is deformed in the differentiated form. In these cases, the measurement should be repeated, or one should wait for the measurement until the apparatus becomes stable. Especially in the case of an FTIR, a measurement requires a rather long time because the Fourier transformation takes a long time. Thus in the case of an FTIR, repetitive measurements greatly lowers the measurement efficiency. Under certain circumstances, the sample does not allow a repetition of the measurement.
The present invention addresses the problem, and one of the objects is to provide an FTIR in which deformation of the interferograms or intrusion of noise in them is adequately avoided, and a reliable absorption spectrum can be obtained.
According to the present invention, a method of obtaining a spectrum in a Fourier transformation infrared spectrophotometer includes the following steps:
sampling a set of data while a movable mirror of the Fourier transformation infrared spectrophotometer performs a reciprocal movement, wherein the set of data constitutes an interferogram;
judging whether the interferogram is reliable or not by comparing the shape of the interferogram with a shape of another interferogram or shapes of other interferograms obtained through neighboring measurement of measurements;
accumulating the data of interferograms that are judged to be reliable; and
constituting an absorption spectrum using a Fourier transformation method based on the accumulated data.
In other words, a Fourier transformation infrared spectrophotometer according to the present invention includes:
judging means for judging whether a first interferogram obtained through a first measurement is reliable or not by comparing the shape of the first interferogram with a shape of another interferogram or shapes of other interferograms obtained through neighboring measurement or measurements;
accumulating means for accumulating data of interferograms that are judged to be reliable by the shape judging means; and
spectrum constituting means for constituting an absorption spectrum using a Fourier transformation method based on the data accumulated by the accumulating means.
In the FTIR of the present invention, all the data of interferograms sampled through measurements are not accumulated, but only data of such interferograms whose shapes are judged to be reliable are accumulated. Data of interferograms that are judged to be unreliable are not included in the accumulation.
In one aspect of the present invention, the shape of an interferogram is judged to be reliable when the noise level of the interferograms is larger than the threshold value. In another aspect of the present invention, the shape of an interferogram is judged to be reliable when a dissimilarity value among the interferograms is larger than the threshold value. Of course, both the noise level and the dissimilarity value may be used in the judgment. Various specific methods of the judgment are described in the description of a preferred embodiment that follows.
Preferably, the judging means compares two consecutively sampled interferograms. In this case, shapes of the chronologically closest interferograms are compared, so that stabler judgment can be made. This is especially useful for judging the stability of the light source or of the detector which becomes stabler as time passes.
Since, in the FTIR of the present invention, unreliable data are adequately avoided from the data accumulation, the reliability of the accumulated data is assured, and an accurate absorption spectrum can be obtained. Thus, even when a mechanical disturbance or an electrical shock affects the measurement instantaneously, there is no need to repeat the measurement, and the absorption spectrum obtained after the measurement is still reliable. This is especially useful when the sample does not allow a second measurement.
The following measurement is possible in the present invention. When an unstable sample whose absorption spectrum changes as time is measured, data accumulation is not performed while the interferograms are judged unreliable, and data accumulation is performed after the sample becomes stable. Thus the absorption spectrum obtained from the FTIR of the present invention assures and reflects the state of the stable sample. Similarly, while the humidity or content of the carbon dioxide in the sample cell or sample chamber is unstable, the sampled data is not included in constituting the absorption spectrum. This allows an automated measurement in which a proper measurement automatically starts when the purge of vapor or carbon dioxide from the sample cell is adequately completed.